1. Field of the Invention
The present invention is related to a method of and an apparatus for forming halftone dots in a halftone image, which is recorded on photosensitive film or the like.
2. Description of the Prior Art
In general, halftone dots in a halftone image are formed as a result of comparison between image data and threshold data referred to as "screen pattern" data. The image data are compared with the screen pattern data with respect to each pixel in an image represented by the image data, whereby a dot signal indicating whether or not each pixel is to be solid is produced. The halftone dots are formed of solid pixels which are exposed as a function of the dot signal.
The screen pattern data are so prepared that the size of a halftone dot increases on a positive halftone image as the density of the original image increases. Therefore, one set of screen pattern data are usually assigned to a full area of a halftone dot at the halftone-dot area rate of (hereinafter referred to as "full halftone-dot area"). If each of the screen pattern data assigned to each pixel in the full halftone-dot area has a different value from each other, the number of tone gradation levels represented by the screen pattern data is equal to the number of pixels in the full halftone-dot area.
On the other hand, at least one technique, such as one disclosed in Japanese Patent Laying Open Gazette No. 63-97067, has been proposed in order to attain a greater number of tone gradation levels while maintaining the size of the full halftone-dot area, that is, while maintaining a screen ruling. This technique employs a combined area which consists of a plurality of full halftone-dot areas, and combined screen pattern data whose values are different from each other and which are assigned to the pixels in the combined area. For example, if the combined area consists of two full halftone-dot areas each of which is a 8.times.8 pixel matrix, the combined screen pattern data represent 128 levels of tone gradation.
A halftone image produced with the combined screen pattern data, however, may cause the following problem. In a part of a halftone image where density is uniform, a fixed shape of halftone dots is repeated because the combined screen pattern data are assigned repeatedly on an image. FIGS. 1A and 1C illustrate enlarged views of a highlight area and a shadow area thus formed, respectively. The highlight area usually has the halftone-dot area rate of at most 10%, and the shadow area usually has a halftone-dot area rate of at least 90%. The highlight area of FIG. 1A includes a first solid portion consisting of one solid dot and a second solid portion consisting of two solid dots. The shadow area in FIG. 1C similarly includes a few types of blank portions. The solid portions and the blank portions are formed repeatedly in respective areas. Consequently, the array of the solid portions and that of the blank portions in FIGS. 1A and 1C can be seen as grid-like patterns as shown in FIGS. 1B and 1D, respectively, with the naked eye. Since these grid-like patterns do not exist in an original image, they decrease the quality of the halftone image. This problem is important especially in the highlight area and the shadow area.